1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to attaching a compressor blade to a rotor disk.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A turbomachine includes a rotor disk with a plurality of rotor blades secured to the disk and spaced circumferentially around the disk. In some turbo-machines, the rotor disk rotates with such a high speed that the centrifugal forces are very high and tend to pull the blade out from the disk. Therefore, the structure in which the blade is secured to the rotor disk is very important.
Another problem with turbo-machines is the replacement of a damaged blade. It is desirable to make the removal and replacement of a damaged blade an easy process in order that the turbomachine can be brought back into operation with a minimal down time. A fan blade assembly used in a gas turbine engine can be an integrally bladed rotor (IBR) in which the rotor disk and the fan blades extending from the rotor are all made as a single piece. The benefits of an IBR is a lighter weight fan blade assembly and the elimination of compressed air leakage through gaps formed between a dovetail slot of the fire tree root of an individual blade attached to the slot. A rotor disk with individual slots for securing the blades to the rotor disk must be massive enough to hold the blades to the disk. This type of rotor disk is heavier than an IBR that compresses the same amount of air. The disadvantage of an IBR is replacing a damaged blade. Since the entire IBR is made of one piece, any damaged blade could result in the entire IBR to be discarded.
One prior art blade attachment method is shown in U.S. Pat. No. 5,129,786 issued to Gustafson on Jul. 14, 1992 and entitled VARIABLE PITCH FAN BLADE RETENTION ARRANGEMENT which discloses a fan blade attached to a disc arm by circular shaped pins secured within first and second seating grooves formed in the blade root and the disc arm opening. One problem with the Gustafson invention is that the circular retaining pins cannot withstand very high shear stress that would result in a turbomachine such as a compressor that operates at high rotational speeds. Another problem with the Gustafson invention is that the resulting force of the fluid acting on the surface of the blade will cause the blade root portion to bend within the supporting opening in the disc arm. In the Gustafson invention, because the retaining pins do not follow the outline of the airfoil surface, the airfoil bending load does not transfer directly to the shear pin.
Another prior art blade retaining method is shown in U.S. Pat. No. 2,974,924 issued to Rankin on Mar. 14, 1961 and entitled TURBINE BUCKET RETAINING MEANS AND SEALING ASSEMBLY which discloses a turbine blade (bucket) attached to the rotor disk by pins fitted within slots on the sides of the blade and the opening of the rotor disk. Four pins for each blade are used, with two pins on each side of the blade root, and where the two pins on the side are angled or offset along a straight line from each other. This offset arrangement of the retaining pins will support the shear loads from the bending force acting on the airfoil surface more than in the above cited Gustafson invention, but still not like the present invention. also, Rankin discloses the retaining pins to be circular or round in cross sectional shape, but also discloses that the pins can have a square cross section (see column 2, line 60).
It is therefore an object of the present invention to provide for a blade retaining method in which the blade can be easily removed and inserted into the rotor disk.
It is another object of the present invention to provide for a retaining pin used to secure a rotor blade within a rotor disk that can withstand high shear stress levels and therefore allow for higher rotational speeds for the bladed rotor disk.
It is another object of the present invention to provide for a blade retaining method in a rotor disk that will transfer more of the airfoil bending force to the shear pin more than any of the cited prior art references.
It is also another object of the present invention to provide for a rotor blade secured to a rotor disk using the well known fir tree configuration that includes all of the above objectives.